A number of techniques for creating a variety of patterns on the surface and inside of transparent substrates using pulsed laser radiation are well known.
One publication disclosing such techniques is the Russian invention document #321422 to Agadjanov et al., published on Nov. 16, 1970 (#140454529-33). The disclosed invention concerns a method of manufacturing decorative products inside a transparent material by changing the material structure by laser radiation. As disclosed, by moving a material relative to a focused laser beam, it is possible to create a drawing inside the material.
U.S. Pat. No. 4,092,518 to Merard discloses a method for decorating transparent plastic articles. This technique is carried out by directing a pulsed laser beam into the body of an article by successively focusing the laser beam in different regions within the body of the article. The pulse energy and duration is selected based upon the desired extent of the resulting decorative pattern. The effect of the laser is a number of three dimensional “macro-destruction” (fissures in the material of the article) appearing as fanned-out cracks. The pattern of the cracks produced in the article is controlled by changing the depth of the laser beam focus along the length of the article. Preferably, the article is in the form of a cylinder, and the cracks are shaped predominantly as saucer-like formations of different sizes arranged randomly around the focal point of the optical system guiding a laser beam. The device used to carry out this technique is preferably a multi-mode solid-state, free-running pulse laser used in conjunction with a convergent lens having a focal length from 100 to 200 mm.
U.S. Pat. No. 4,843,207 to Urbanek et al. discloses a method of creating controlled decorations on the surface of a hollow symmetrical transparent article. This technique is preferably carried out on glass. The glass is preconditioned with a coating on the outer surface of the glass being approximately 1.2 mm thick and made of a material having at least 75% absorption of laser radiation. The technique is also carried out using a laser having a wave of length of 0.5 to 2 microns acting upon the external coating through the wall of the cylindrical glass article. The laser beam moves so that it is focused on the surface of the cylinder, and moves about the axis of symmetry of the cylinder to irradiate the aforementioned surface coating. As a result, the irradiated portions of the surface coating go through a phase change and a pattern is formed.
True interior laser-etching of a transparent article is described in U.S. Pat. No. 5,206,496 to Clement et al. This patent discloses a method and apparatus for providing in a transparent material, such as glass or plastic, a mark which is visible to the naked eye or which may be “seen” by optical instruments operating at an appropriate wavelength. The Clement et al. patent describes a method and apparatus for producing a subsurface marking which is produced in a body such as bottle by directing into the body a high energy beam and bringing the beam to focus at a location spaced from the surface, so as to cause localized ionization of the material. In the preferred embodiment the apparatus includes a laser as the high energy beam source. The laser may be a N-YAG laser that emits a pulsed beam of laser radiation with a wavelength of 1.06 microns. The pulsed beam is incident upon a first mirror that directs the beam through a beam expander and a beam combiner to a second mirror. A second source of laser radiation in the form of a low power He—Ne laser emits a secondary beam of visible laser radiation with a wavelength of 638 nm. The secondary beam impinges upon the beam combiner where it is reflected toward the second reflecting surface coincident with the pulsed beam of laser radiation from the Nd-YAG laser. The combined coincident beams are reflected at the reflecting surface via reflecting two other surfaces to a pair of movable mirrors for controlling movement of the beam. The beam then passes through a lens assembly into the body to be marked.
U.S. Pat. No. 5,575,936 to Goldfarb discloses a process and apparatus where a focused laser beam causes local destruction within a solid article, without effecting the surface thereof. The apparatus for etching an image within a solid article includes a laser focused to a focal point within the article. The position of the article with respect to the focal point is varied. Control means, coupled to the laser, and positioning means are provided for firing the laser so that a local disruption occurs within the article to form the image within the article.
European Patent Application No. 624421A2 to Erokhin (priority based upon U.S. patent application Ser. No. 08/110,048 and Russian Patent Application No. 29023) discloses a technique which depends on a particular optical system including a diffraction limited Q-switched laser (preferably a solid-state single-mode TEM00) aimed into a defocusing lens having a variable focal length to control the light impinging on a subsequent focusing lens that refocuses the laser beam onto the transparent article being etched. The laser power level, operation of the defocusing lens, and the movement of the transparent article being etched are all controlled by a computer. The computer operates to reproduce a pre-programmed three-dimensional image inside the transparent article being etched. In the computer memory, the image is presented as arrays of picture elements on various parallel planes. The optical system is controlled to reproduce the stored arrays of picture elements inside the transparent material.
In the prior art arrangements, neither the characteristics of the transparent material or physical phenomenon of laser-matter interaction are taken into consideration. As a result, the distance between image etch point elements is selected with no consideration for the material strength. Therefore, the article may be damaged if the elements are too close to each other. On the other hand, the image resolution may be unacceptably low if the elements are too far from each other.
The first computer graphic system for producing an image inside optically transparent material which takes into account the characteristics of the transparent material is described in U.S. patent application Ser. No. 08/643,918 to Troitski et al. An image reproducible inside optically transparent material is defined by potential etch points, in which the breakdowns required to create the image in the selected optically transparent material are possible. The potential etch points are generated based on the characteristics of the selected optically transparent material. If the number of the potential etch points exceeds a predetermined number, the system carries out an optimization routine that allows the number of the generated etch points to be reduced based on their size. To prevent the distortion of the reproduced image due to the refraction of the optically transparent material, the coordinates of the generated etch points are adjusted to correct their positions along a selected laser beam direction.
Images comprising laser-generated etch points produced by all known systems and methods generally suffer from having a lower resolution than the real image (such as a picture) or the computer processing image from which it is derived. As such, some of the details of the original image are lost. It is known that the number of pixels covering the area of the image determines the resolution of the computer image. On the other hand, the resolution of the reproducible image is determined by the number of its etch points. A value “d” comprises a minimum distance between adjacent etch points in optically transparent material for a selected energy of laser radiation which avoids an internal split or other undesirable damage or weakening of the material. Therefore, such a system should produce adjacent etch points in the same plane separated by a distance greater than d, as described in U.S. patent application Ser. No. 08/643,918 to Troitski et al.
An image reproduced in an object may be derived from a computer-generated image of the object. Adjacent pixels of a computer processing image touch each other. Not all pixels, but only part of them (usually less than ¼ of their number), can be corresponding etch points in reproducible image if one is to avoid their being spaced closer than distance d. On the other hand, the greater the number of pixels which are lost, the lower the resolution of the reproduced image in comparison to the computer image.
Another problem is that for typical values of d, a viewer of the-object may discern the individual etch points formed in the material. As a result, an image reproducible in optically transparent material comprising widely spaced etch points appears rough visual to the naked eye (see FIG. 2). This roughness is generally undesirable, and may be unacceptable for portrait and similar images requiring high resolution.
The inability to transform all image pixels into etch points in material has a profound effect on the quality of a reproduced image, such as a portrait. In addition, however, any inexact transformation of gray shades can also make a significant difference in quality of the produced image. One method of transforming the gray shades of a displayed image into gray shades of an image reproduced inside of material, using corresponding sizes of etch points created by adjusting the laser radiation power, has been described in European Patent Application No. 624421 A2 to Erokhin. The disclosed method permits only the reproduction of a “half-tone picture” with considerable fluctuation in the appearance of the gray shades. This is due in part to the face that there is considerable fluctuation in the appearance of etch points produced by even the same laser energy. Consequently, to produce a portrait or other image with high quality it is very important to produce etch points with the same shape. In order for the etch points to have a high brightness, it is desirable for the etch points to have the shape of a sphere.
Further, the number of points required to produce a portrait with a required level of detail increases when producing a 3-D image. For instance, FIG. 2 shows a portrait of Abraham Lincoln which has 5 times as many points than the 3-D image of the Lincoln Memorial, shown in the same figure.
Another problem is that it is often desirable to produce several duplicate works of a particular image or portrait in different objects. Known laser systems which are used to produce an image inside of an optically transparent material use only one beam, treat only one article and make only one etch point for one pulse of the laser. It is an object of the invention to create a laser system that allows for the production of multiple portrait images, simultaneously creating etch points inside every image at the same time.
It is also an object of the invention to provide a system and method which permits the production of an image, such as a portrait, inside optically transparent material with a number of points or pixels and gray shades (i.e. the same resolution) similar to the computer processing image.
It is also an object of the invention to provide a method and apparatus for generating etch points inside a material which are other than shades of gray, such as iridescent (i.e. etch points having a multi-color appearance as a result of their expansion of white light into a color spectrum).
It is also an object of the invention to provide a laser-computer graphics system for creating a sculpture or the like which combines methods of production 3-D images and portrait images, so that each viewer observing the sculpture sees the image with a similar resolution to the computer image and without a defined point structure.